The invention relates to a process for synchronizing the scanning circuit of an image display device, said images having been acquired by a camera having a scanning circuit controlled by a given acquisition clock.
The process is applicable to videophone, videophone conference, television and remote monitoring equipment.
In an analog video transmission, synchronization pulses are regularly inserted in the image signal. They indicate the starts of images and the starts of lines.
The display device or monitor in the case of videophony, is synchronized with said reference pulses, which enables it to display the start of the image at the top left and the starts of the lines along the left-hand edges. Therefore an analog video signal contains two reference frequencies, namely the line frequency and the frame frequency.
However, in the case of the transmission of analog signals on a wideband network in the case of videophony, that there was a scrolling of a shadow superimposed on the displayed scene. Thus, in analog transmission on the wideband network in videophony (FIG. 1), the video signal from the camera A reaches the monitor B without any significant deformation and this also applies to the signal from camera B to monitor A. The scanning circuits of the monitor B are therefore controlled by the signal from the camera A. However, there is no reason why the scans of the camera B are identical to those of the camera A. The camera and monitor B of a videophone or picture phone are located very close to one another and the influences by radiation of their scanning circuit lead to said scrolling. The speed of the latter corresponds to the frequency shift between the two scans.
In the case of a digital video transmission, where the signals have been compressed by a coder in order to be transmitted, these line and frame frequencies are no longer transmitted as regular, periodic patterns.
By eliminating space and time redundancies by known image compression methods, the number of bits per image becomes variable. Code words characterizing the starts of images are transmitted, but they are no longer separated by equal times and no longer constitute a clock cycle with which the reception device can be synchronized. The line synchronization may no longer be transmitted.
The reception device making it possible to display these images has a decoder making it possible to reconstruct the originally compressed images, so as to record entire images in display storage means, but at variable times dependent on the number of bits transmitted per image and the power of the decoder.
Therefore the scan synchronization problems have not hitherto been solved in a satisfactory manner.